For error control, the fourth-generation (4G) wireless system known as “Long-Term Evolution (LTE), standardized by members of the 3rd-Generation Partnership Project (3GPP), uses hybrid-ARQ (HARQ). After receiving downlink data in a subframe, the mobile terminal attempts to decode it and report, via a Physical Uplink Control Channel (PUCCH), to the base station whether (ACK) or not (NACK) the decoding was successful. In the event of an unsuccessful decoding attempt, the base station (eNodeB or eNB) can retransmit the erroneous data. Similarly, the base station can indicate to the mobile terminal whether the decoding of the Physical Uplink Shared Channel (PUSCH) was successful (ACK) or not (NACK) via the Physical Hybrid ARQ Indicator Channel (PHICH).
In addition to the hybrid-ARQ ACK/NACK information transmitted from the mobile terminal to the base station, uplink control signaling from the mobile terminal to the base station also includes reports related to the downlink channel conditions, referred to generally as channel-state information (CSI) or channel-quality information (CQI). This CSI/CQI is used by the base station to assist in downlink resource scheduling decisions. Because LTE systems rely on dynamic scheduling of both downlink and uplink resources, uplink control-channel information also includes scheduling requests, which the mobile terminal sends to indicate that it needs uplink traffic-channel resources for uplink data transmissions.
Packet data latency is one of the performance metrics that vendors, operators and also end-users (via speed test applications) regularly measure. Latency measurements are done in all phases of a radio access network system lifetime, e.g., when verifying a new software release or system component, when deploying a system, and when the system is in commercial operation.
Improved latency compared to previous generations of 3GPP radio access technologies (RATs) was one performance metric that guided the design of LTE. LTE is also now recognized by its end users to be a system that provides faster access to internet and lower data latencies than previous generations of mobile radio technologies.
Packet data latency is important not only for the perceived responsiveness of the system, but is also a parameter that indirectly influences the throughput of the system. HTTP/TCP is the dominating application and transport layer protocol suite used on the Internet today. According to HTTP Archive (http://httparchive.org/trends.php), the typical size of HTTP based transactions over the Internet is in the range of a few tens of kilobytes up to 1 Mbyte. In this size range, the Transport Control Protocol (TCP) slow-start period is a significant part of the total transport period of the packet stream. During TCP slow start, the performance is latency limited. Hence, improved latency can rather easily be shown to improve the average throughput for this type of TCP based data transactions.
Radio resource efficiency can also be positively impacted by latency reductions. Lower packet data latency can increase the number of transmissions possible within a certain delay bound; hence, higher block-error rate (BLER) targets could be used for the data transmissions, freeing up radio resources and potentially improving the capacity of the system. It should also be noted that reduced latency of data transport may also indirectly give faster radio control plane procedures like call set-up/bearer set-up, due to the faster transport of higher layer control signaling.
There are several current applications that will be positively impacted by reduced latency, in terms of increased perceived quality of experience. Examples are gaming and real-time applications like Voice over LTE/Over-the-top voice over IP (VoLTE/OTT VoIP) and multi-party video conferencing. In the future, there will be a number of new applications that will be more delay critical. Examples may be remote control/driving of vehicles, augmented reality applications in, e.g., smart glasses, or specific machine communications requiring low latency.
LTE is a radio access technology based on radio access network control and scheduling. These facts impact the latency performance since a transmission of data need a round trip of lower layer control signaling. An example of this lower layer control signaling is shown in FIG. 1. The data is created by higher layers at T0. Then, the user equipment (UE) sends a scheduling request (SR) to the eNB to obtain resources for sending the data to the network. The eNB processes this SR and responds with a grant of uplink resources. After that, the data transfer can start, as shown at T6 in the figure.
When it comes to packet latency reductions, one area to address is the reduction of transport time of data and control signaling, e.g., by addressing the length of a transmit-time-interval (TTI), and the reduction of processing time of control signaling, e.g., by reducing the time it takes for a UE to process a grant signal.